It is proposed to devise convenient syntheses for a wide variety of spin-labelled derivatives of adenosine and guanosine and their nucleotides and cyclic nucleotides. Such compounds would be invaluable in ESR studies of enzymes for which ATP, ADP, GTP, GDP, cyclic AMP and cyclic GMP are substrates or effectors. The variety of nucleosides and nucleotides for which syntheses are to be devised include compounds with the spin-label attached at each of the three carbon atoms of the purine ring, compounds with the spin label linked to the purine ring through nitrogen, sulfur and oxygen, and compounds with variations on the immediate environment of the nitroxyl group. From such a variety of compounds it should be possible to choose an efficient substrate for almost any enzyme. The spin labelled compounds will be tested for activity with enzymes and cells of different specificities, and will be used as spin-labels in studies of the active site of glutamine synthetase and in studies of the adensine and ADP binding sites of blood platelet membranes.